Cytochrome A-type

Takai etal. (2001) have obtained from A. ferrooxidans TI-1, a cytochrome a-type iron oxidase. The enzyme catalyzes the oxidation of ferrous ion at pH 3.0 and at 45°C in vitro V for oxygen consumption is 13.8 pmol mg 1 min-1 (242 mol Fe2+s 1). As the enzyme has neither cytochrome c nor cytochrome b, the ferrous ion-oxidizing pathway is constituted of only cytochrome a in the bacterium. Cytochrome c oxidase purified from a mercury-resistant strain of A. ferrooxidans was shown to reduce mercuric ion to elemental mercury (Sugio et al., 2001). 

Fig. 19.2 The heme group Type A hemes are found in cytochrome a Type B hemes are found in hemoglobin, myoglobin, peroxidase, and cytochrome b Type C hemes are found in cytochrome c chlorohemc is found in chlorocruorin.

Complex IV 200 kDa (probably as a dimer) 13 2 A type hemes (a, aj) 2 or 3 Cu Spans inner membrane, cytochrome c site on outer face 0.16-1.00 Pumps protons out of matrix during electron transport/4e"... 

Cytochrome P450-type monooxygenase systems, which have a generally low substrate specificity, are widely distributed in the species of fish used for toxicity testing (Funari et al. 1987). 

Postgate (51) observed a high concentration of a type of cytochrome, designated cytochrome cz, in the strictly anaerobic sulfate-reducing bacterium, Desul/ovibrio. The cytochrome Cz Desulfovibrio vulgaris has been shown to possess a very low redox potential (—0.205 V), two heme groups per mole and a sequence which implies two half sections, each of which binds a heme group (52). 

Cytochrome 6554 Type A flavoprotein (FprA) Flavoprotein/flavodoxin... 

Extensive analysis of the EPR and redox behavior of this unusual copper protein led to the hypothesis that the protein might contain a Cu(A) site similar to that in cytochrome oxidase (Riester et ai, 1989) and that the unusual seven-line EPR is due to the Cu(A)-type site. An alternative interpretation of this EPR is based on electron spin-echo spectroscopy as well, and that is that the seven-line EPR is due to a half-met Cu—Cu pair and to unusual type I sites (Jin et ai, 1989). Three sets of spin-echo peaks can be attributed to nitrogens on imidazole ligands to a CuA-type site and to another imidazole on the half-met site. The electron spin-echo spectra of cytochrome oxidase are similar, although there is not enough copper in cytochrome oxidase for a half-met site. Conceivably, the property of delocalization of the paramagnetic electron could be effected by the proposed bridging between Cub and heme as (nomenclature summarized by Capaldi, 1990), which are proposed to be 3-4 A apart. 

The major product of nitrite reduction is NO (Denariaz et ai, Iwasaki and Matsubara, 1972 Kakutani et al., 1981 Liu et ai, 1986 Masuko et ai, 1984 Sawada et ai, 1978 Zumft et ai, 1987) just as is the case with the cytochrome cd, type enzyme. As discussed previously in this chapter, the Cu-type enzyme from A. cycloclastes has been reported to reduce nitrite to N2O in the presence of NO (Averill and Tiedje, 1990 Jackson et ai, 1991), a catalytic capability apparently not exhibited by heme-type nitrite reductases. 

A possible model for the cytochrome cd, type, NO-producing nitrite reductases is the reaction between nitrite and Hb (Doyle et ai, 1981, 1984). This reaction, which produces Hb and HbNO as products, can be written as occurring in two steps [Eq. (5)]. The second step is simply the complexation of NO by Hb. 

In the reduced state, cytochromes P-450 may also bind certain ligands to give particular difference spectra. The most well known is that which occurs when carbon monoxide binds giving an absorption maximum at 450 nm. A type III spectrum gives two peaks at 430 and 455 nm after binding of certain compounds such as ethyl isocyanide or methylenedioxyphenyl compounds to the reduced enzyme. The latter form stable complexes with the enzyme and are also inhibitors. 

The heme cofactors of a and b cytochromes are tightly, but not covalently, bound to their associated proteins the hemes of c-type cytochromes are covalently attached through Cys residues (Fig. 19-3). As with the flavoproteins, the standard reduction potential of the heme iron atom of a cytochrome depends on its interaction with protein side chains and is therefore different for each cytochrome. The cytochromes of type a and b and some of type c are integral proteins of the inner mitochondrial membrane. One striking exception is the cytochrome c of mitochondria, a soluble protein that associates through electrostatic interactions with the outer surface of the inner membrane. We encountered cytochrome c in earlier discussions of protein structure (see Fig. 4-18). 

The most numerous and most complex monooxygenation reactions are those employing a type of heme protein called cytochrome P-450. This cytochrome is usually present in the smooth ER rather than the mitochondria. Like mitochondrial cytochrome oxidase, cytochrome P-450 can react with 02 and bind carbon monoxide, but it can be differentiated from cytochrome oxidase because the carbon monoxide complex of its reduced form absorbs light strongly at 450 ran—thus the name P-450. 

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